There are many known compositions for application to the human skin to protect it from the burning radiation of natural sunlight. Much research has taken place to determine the components of sunlight radiation which produce the phenomenon known as "sunburn" or erythema. Although there is some disagreement regarding the exact limits of the range of ultra-violet wavelengths which produce erythema, it has generally been established that wavelengths below about 3150 A, particularly in the range of from about 2900 A to about 3150 A produce the erythema. The peak erythema producing wavelength may vary, depending upon the individual exposed; however, for most humans the peak is about 2967 A. It has also been discovered that the ultra-violet component of sunlight causes the human skin to tan. Although tanning is not fully understood, it is believed to be the result of the formation of melanin, a skin pigment and a migration of this melanin to the surface of the skin and the oxidative darkening thereof. It has been suggested that tanning is nature's method of protecting against the erythema producing regions of the sunlight radiation since the obtention of a good tan prevents erythema or burning of the skin without the necessity of resorting to suntan or sun-screen agents.
It is also believed by many people that tanning produces a healthy physiological effect on the human skin. There are also psychological benefits to be derived from a so-called "healthy tan" which is looked upon as a status symbol in many parts of the world.
Many suntan or sun-screen agents have been proposed in the past to protect the human skin from the erythema producing effects of sunlight. Unfortunately, the burning radiation, namely, that having a wavelength of from about 2900 to about 3150 A also contains much of the primary tanning radiation. Accordingly, the sun-screen agents proposed to date screen out the primary tanning as well as the burning radiation rendering it a time-consuming process for obtaining a good tan.
For many years it was thought that tanning was produced only by the same range of wavelengths that produced erythema. Recently, however, tanning has been shown to be produced by longer wavelengths ranging in some cases as high as about 6000 A.
It has recently been shown that radiation of longer wavelengths than about 3150 A produces tanning without erythema. There is not a general agreement as to the exact range of this secondary tanning radiation since there is considerable variation in the tanning sensitivities of various individuals and considerable variation in the effectiveness of different wavelengths for tanning various individuals. For average skin, however, it appears that tanning without erythema is produced in a range of from about 3150 to 4000 A with a broad peak at about 3600 A. Ultra-violet radiation at these wavelengths produces the maximum amount of tanning which can be obtained without the concurrent production of erythema. As noted above, however, this is a secondary or incidental tanning range of far less intensity than that of the lower wavelengths. Its advantage lies in the fact that tanning is produced, albeit slowly, without erythema or burning.
Included among the sun-screening agents suggested to date are benzyl salicylate, menthyl salicylate, glyceryl monosalicylate, umbelliferone acetic acid, quinine oleate, the various esters of p-amino benzoic acid, derivatives of beta-resorcylic acid, derivatives of cinnamic acid, derivatives of benzophenone, etc.
It is important to note the distinction between the "sunscreen" and "suntan" preparations which have been previously proposed to protect the skin from erythema and promote tanning. An ideal "sunscreening" agent would be opaque to all erythemal and tanning radiation, while an ideal "suntanning" agent would be opaque only to the erythemal radiation (or a controlled amount thereof) and would pass all of the nonerythemal or tanning radiation. Conventional "sun-tanning" agents pass about 10-20% of incident erythemal radiation and more than 90% of the longer wavelength tanning radiation. The "sun-screen" agents attempt to screen out more than 90% of the incident erythemal radiation and most of longer tanning wavelengths as well.
It is an object of the present invention to provide a sun-screening agent and composition which goes one step further than those of the prior art and, (1) screens out the erythema producing ultra-violet radiation of natural sunlight, (2) permits the incident tanning radiation having wavelengths above the erythema producing radiation to pass through and (3) enhances the tanning of the human skin to which it is applied by making available thereto more tanning radiation than those sun-screens heretofore available.